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Bureau of Mines Information Circular/1987 





Natural and Synthetic Zeolites 



By Robert A. Clifton 




UNITED STATES DEPARTMENT OF THE INTERIOR 




Information Circular 9140 



Natural and Synthetic Zeolites 



By Robert A. Clifton 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 







4"i v « 



Library of Congress Cataloging-in-Publication Data 



Clifton, Robert A. 
Natural and synthetic zeolites. 

(Information circular ; 9140) 

Bibliography: p. 20 

Supt. of Docs, no.: I 28.27: 9140 

1 . Zeolites. I. Title. II. Series: Information circular (United States. Bureau of Mines) ; 9140 



TN295.U4 [QE391.Z5] 622 s [549'.68] 86-600409 



CONTENTS 



Page 

Abstract 1 

Introduction 1 

Definitions 2 

Structure and classification 2 

Formation processes and geologic occurrence 5 

Saline, alkaline lakes 5 

Saline, alkaline soils 5 

Marine sediments 7 

Open hydrologic systems 7 

Hydrothermal systems 7 

Burial diagenetic systems 7 

Magmatic systems 8 

Impact craters 8 

Exploration 8 



Page 

Synthesis 10 

Chemical and physical modification 12 

Dehydration and rehydration 12 

Structural hydroxyl groups 12 

Sorption and diffusion 13 

Molecular sieving 13 

Diffusion 14 

Pore volume 14 

Zeolites and catalysis 15 

Economic considerations 16 

Applications 16 

Summary 19 

References 20 



ILLUSTRATIONS 



1. Basic structural units of zeolites 2 

2. Examples of secondary building units 2 

3. Zeolite framework polyhedra 3 

4. Common depictions of zeolite structure: sodalite alpha cage 3 

5. Possible pathways for formation of faujasite and A-type zeolites 4 

6. Polyhedra orientation and zeolite structure 4 

7. Channels in zeolites 4 

8. Effect of aperture size on catalytic properties 5 

9. Patterns of authigenic zeolites and feldspars 6 

10. Ancient Lake Tecopa near Shoshone, CA, showing diagenetic facies 7 

11. Zonal distribution of zeolites and silicates in burial diagenesis 8 

12. Structure of ZSM-5 12 

13. Channel system of ZSM-5 12 

14. Aluminum atom concentration and SiCv A1 2 3 ratio for various zeolites 12 

15. Distribution of pore sizes in microporous adsorbents 13 

16. Kinetic diameters of some simple molecules 14 

17. Adsorption of cyclohexane and water on dealuminized mordenite 17 

18. Fluidized-bed methanol-to-gasoline process 18 

19. Combined fluid-gas adsorption cooling and heating system utilizing zeolites 18 



TABLES 



1. Dates of discovery of 40 zeolites 1 

2. Free diameters of apertures governing access to channels 3 

3. Zeolite structural groups 5 

4. Discovery and/or initial commercial investigations of prominent zeolite deposits in the United States 9 

5. Selected summary of unsubstantiated zeolite syntheses 10 

6. Synthetic zeolites: Na 2 0-Al 2 03-Si0 2 -H 2 system 11 

7. Summary of zeolite dehydration behavior 13 

8. Effect of cation exchange on void volume in zeolite A 14 

9. Void volume of zeolite NaX at25° C 15 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


A 


angstrom 


kJ/L 


kilojoule per liter 


A 3 


cubic angstrom 


km 


kilometer 


atm 


atmosphere 


lb 


pound 


Btu/gal 


British thermal unit per gallon 


m 


meter 


°C 


degree Celsius 


lam 


micrometer 


cm 3 


cubic centimeter 


mol pet 


mole percent 


°F 


degree Fahrenheit 


ppm 


part per million 


ft 


foot 


St 


short ton 


g 


gram 


wt% 


weight percent 


g/cm 3 


gram per cubic centimeter 


yr 


year 


h 


hour 







NATURAL AND SYNTHETIC ZEOLITES 

By Robert A. Clifton' 



ABSTRACT 

This Bureau of Mines report discusses the mineralogy of zeolites, the properties that make 
them commercially valuable, the conditions of formation of both natural and synthetic zeolites, and 
present and potential markets. 



INTRODUCTION 



Zeolites are crystalline aluminosilicates of the alkaline 
and alkaline-earth metals. They possess many desirable ion- 
exchange, molecular sieving, and catalytic properties, which 
make them valuable mineral commodities (l). 2 Synthetic 
zeolites have been used for over 25 yr in commercial proc- 
esses, but only recently have natural zeolites been viewed as 
potentially valuable mineral commodities (1). Because of this 
interest, the Bureau of Mines undertook this report to 
discuss the occurrence, synthesis, mineralogy, economics, 
and uses of these versatile minerals. 

Zeolites were first identified by Constedt in 1756 (2). 
The name zeolite, from the Greek words meaning "boiling 
stone," alludes to the frothing and bubbling observed by 
Cronstedt when he heated several crystals. In 1845, Way (3) 
discovered that certain soils retained ammonium salts. 
Breck (1) reported that hydrated silicates in the soil were 
found to be responsible and that these were probably the 
first ion-exchange experiments. Weigel and Steinhoff (U), in 
1925, were the first to determine that chabazite selectively 
absorbed smaller organic molecules and rejected large 
molecules. This phenomenon was described by McBain (5) in 
1932 as "molecular sieving." It was not until the 1940's and 
1950's that research on the properties of zeolites increased 
dramatically, especially at the laboratories of Barrer at the 
Imperial College in London (6). Barrer's work included the 
recognition of various molecular sieve types, quantitative 
studies of molecular sieving, and the use of ion exchange to 
modify the exchange properties of molecular sieves. 

As knowledge of the properties of zeolites increased, it 
became apparent that they could be utilized for industrial 
processes. Zeolites, however, were considered to be 
mineralogical curiosities that filled vugs and fractures in ig- 
neous rocks (6). Large tonnages of natural zeolites were not 
discovered until the late 1950's when Ames, Sands, and 
Goldich (7), Deffeyes (8), and Mumpton (9) reported on vast 
sedimentary deposits in the Western United States. Prior to 
the 1950's, the emphasis was on the synthesis of zeolites for 
commercial use (10). 

The synthesis of zeolites was reported as early as 1862, 
although Breck (11) noted that the early work was not 
substantiated by X-ray diffraction and some of it is not 
reproducible. He credited Barrer with the first synthesis of 
analcime-type zeolites substantiated by X-ray diffraction in 
1951. 



Initially, zeolites were synthesized under temperatures 
and pressures thought to be responsible for the crystalliza- 
tion of zeolites in basaltic rocks (11). Breck reports that in 
1959, Milton and coworkers at Union Carbide Corp. 
developed a new technique that allowed low-temperature 
synthesis of zeolites. Their technique used extremely reac- 
tive components in a closed system and temperatures of 
crystallization more typical of those for organic compounds. 
This procedure was adaptable to large-scale production of 
synthetic zeolites (11). 

By the 1980's, 40 natural zeolites had been identified 
(table 1), over 100 zeolites had been synthesized, and natural 
zeolite resources within the United States were estimated to 
be 10 trillion st (12-1U). Approximately 163,000 st of natural 
and synthetic zeolites were consumed in the United States 
in 1980 (15). Of this, 5,000 st were natural zeolites (16). In 
1985, approximately 13,000 st of natural zeolites were 
mined in the United States (17). While synthetic zeolites 
have been used extensively in commercial applications, de- 
mand for natural zeolites has been limited (16). Their 
primary use is in areas where the use of synthetic zeolites 
would be uneconomical. Several uses, both commercial and 
experimental, include ammonium-ion removal for aqua- 
culture and uranium mine waste water, odor control for 
chicken farming and cat litter, and removal of heavy metal 
ions from nuclear, mine, and industrial waste waters (18). 

Table 1.— Dates of discovery of 40 zeolites 



'Physical scientist, Division of Industrial Minerals, Bureau of Mines, 
Washington, DC (retired). Revised by Robert L. Virta, physical scientist. 
Division of Industrial Minerals. 

italicized numbers in parentheses refer to items in the list of references 
at the end of this report. 



Stilbite 1756 

Natrolite 1758 

Chabazite 1772 

Harmotone 1775 

Analcime 1784 

Laumontite 1785 

Thomsonite 1801 

Scolecite 1801 

Heulandite 1801 

Gmelinite 1807 

Mesolite 1813 

Gismondine 1816 

Brewsterite 1822 

Epistilbite 1823 

Phillipsite 1824 

Levynite 1825 

Herschelite 1825 

Edingtonite 1825 

Faujasite 1842 

Mordenite 1864 



Clinoptilolite 1890 

Offretite 1890 

Erionite 1890 

Kehoeite 1893 

Gonnardite 1896 

Dachiardite 1905 

Stellerite 1909 

Ferrierite 1918 

Viseite 1942 

Yugawaralite 1952 

Wairakite 1955 

Bikitaite 1957 

Paulingite 1960 

Garronite 1962 

Mazzite 1972 

Barrerite 1974 

Cowlesite '1975 

Merlinoite 1976 

Svetlozarite '1976 

Amicite '1979 



1 Proposed (4). 



DEFINITIONS 



Zeolites are crystalline-framework aluminosilicates 
based on a three-dimensional network of Si0 4 tetrahedra, 
with all four oxygens shared by adjacent tetrahedra (Z.9). 
Zeolites may be represented by the empirical formula 
M.„ n OAl L ,0 3 -xSi0 2 yH 2 0. M is an alkali or alkaline earth ca- 
tion of n valence, x is a number between 2 and 10, and y is a 
number between 2 and 7. The principle cations are sodium, 
potassium, magnesium, calcium, strontium, and barium. 
The cations are loosely bound in the structure and may be 
exchanged, to varying degrees, by each other. The frame- 
work contains channels and interconnected voids occupied 
by cations and water molecules. Most zeolites can be rever- 
sibly dehydrated. 

Zeolites are characterized by the following properties 
(11): 

1. High degree of hydration. 

2. Low density and large void volume when dehy- 
drated. 

3. Stability of the crystal structure of many zeolites 
when dehydrated. 



4. Cation exchange properties. 

5. Uniform molecular-sized channels in the dehydrated 
crystals. 

6. Various physical properties such as electrical conduc- 
tivity. 

7. Adsorption of gases and vapors. 

8. Catalytic properties. 

Molecular sieves are materials that, because of their in- 
ternal structure, can selectively adsorb molecules according 
to their size and/or shape (6). All zeolites are molecular 
sieves, but not all molecular sieves are zeolites. Activated 
carbon, activated clays, alumina powder, and silica gels are 
examples of molecular sieves that are not zeolites (11). 

A gel is a hydrous metal aluminosilicate that is prepared 
from either aqueous solutions, reactive solids, colloidal sols, 
or reactive aluminosilicates such as the residue structure of 
metakaolin (derived from kaolin clay by dehydroxylation) 
and glasses (11). 



STRUCTURE AND CLASSIFICATION 



Zeolites are crystalline frameworks of oxygen, 
aluminum, and silicon extending in a three-dimensional 
framework (1). Figures L4 and IB illustrate the basic struc- 
tural building blocks. In both cases, either a silicon or 
aluminum atom is at the center of a tetrahedron formed by 
the oxygen atoms (19). If an aluminum atom is present 
rather than a silicon atom, a positive metal ion is required to 
maintain a charge balance (figs. 1B-1C). 




These tetrahedra are then grouped together to form 
secondary building units (SBU's) (20, p. 10; 21). There are 
eight SBU's: the single 4-ring (S4R), the single 6-ring (S6R), 
the single 8-ring, the double 4-ring (D4R), the double 6-ring 
(D6R), the natrolite unit (4-1), the mordenite unit (5-1), and 
the stilbite unit (4-4-1) (fig. 2). Only the silicon atoms are il- 
lustrated, for clarity. The D4R and D6R units are combina- 
tions of the S4R and S6R units. The SBU's can be assembled 
to form a variety of polyhedra (fig. 3). These polyhedra are 
normally depicted as the "ball and stick" model (fig. 44), the 
"solid tetrahedral" model (fig. 45), and the "polyhedral" 
model (fig. AC). 

The alpha cage or truncated cuboctahedron depicted in 
figure 4C is a component of more than one zeolite. Zeolite A 
is composed of alpha cages joined by D4R's (fig. 5). If these 
cages were joined by D6R's, faujasite or zeolite Y would be 
formed, with entirely different topology, cavity size, chan- 
nel size and number, and different molecular sieving proper- 
ties. 



Figure. 1— Basic structural units of zeolites. A, Tetrahedron 
with silicon atom (filled circle) at center and oxygen atoms 
(open circles) at apexes; S, tetrahedron with aluminum atom 
substituting for silicon and attached monovalent cation com- 
pensating for charge difference between silicon and 
aluminum; C, divalent atom compensating for charge im- 
balance between silicon and aluminum in multiple tetrahedron 
chain. 




Figure 2.— Examples of secondary building units. A, S4R; B, 
D6R; C, natrolite unit (4-1); D, mordenite unit (5-1); E, stilbite 
unit (4-4-1). (Reprinted with permission from Mineralogy and Crystal 
Chemistry of Zeolites (p. 32), by G. Gottardi. Ch. in Natural Zeolites: Oc- 
currence, Properties, Use, ed. by L. B. Sand and F. A. Mumpton. 
Copyright 1978, Pergamon Press.) 




Figure 3.— Zeolite framework polyhedra. A, Chabazite 
20-hedron, capped by hexagonal prisms; B, gmelinite 
14-hedron of type II; C, losod 17-hedron of type II, with 
associated 11-hedral canrlnlte cage; D, erionite 23-hedron; E, 
levynite 17-hedron of type I, with associated hexagonal 
prisms; F, zeolite A 26-hedron of type I; G, faujasite 26-hedron 
of type II. (Reprinted with permission from Zeolites and Clay Minerals 
as Sorbents and Molecular Sieves (p. 38), by R. M. Barrer. Copyright 
1978, Academic Press.) 

Different zeolites with distinctive characteristics and 
properties can also be produced using the same SBU (D6R) 
and the same polyhedra, by changing the orientation of the 
polyhedra with respect to each other (fig.|6). 

Cavities and channels are created within the zeolite 
structure when the SBU's are assembled. There can be as 
many as three channel types: nonintersecting, intersecting 
in two dimensions, or intersecting in three dimensions (11). 
In figure 7 from Beck (11), A and B illustrate the channel 
system in faujasite and zeolites X and Y: A shows the four 
connected networks, and B, the channels as an array of 
overlapping tubes. Figure 1C is a diagram of the three- 
dimensional system of the intracrystalline channels in 
zeolite A. Figure ID is a diagram of the two independent 
sets of channels in paulingite. Both sets are of the same size 
but do not intersect. The rest of the panels in figure 7 are 
schematic illustrations of the main intracrystalline channels 
in zeolites that have frameworks based on parallel arrays of 
6-rings: E, chabazite, F, gmelinite, G, erionite, and H, 
levynite. 



Channel sizes in a given zeolite can vary significantly 
depending on which cation is present, temperature and ther- 
mal vibration, and structural deformations (1, 22). Theo- 
retical calculations based on the number of tetrahedra form- 
ing a channel opening or aperture and assigning to oxygen 
ions the accepted 2.7- A diameter give a good approximation 
of the available channel size (6). Table 2, selected from Bar- 
rer (6), gives these calculated dimensions for ring sizes 
observed in zeolites. Six-ring apertures, present in many of 
the zeolites, and the channels limited by them are omitted 
because these rings allow entry only to the smallest polar 
molecules, such as water. 

The cavities within the zeolite structure are accessed 
through these apertures in the polyhedral framework struc- 
ture. Miller (19) discussed the importance of aperture size, 
using the truncated octahedra of zeolite A, faujasite, and X 
and Y zeolites as examples. Since the apertures of zeolite A 
are too small to allow large molecules to enter the inner 
cavity, these crystals are limited in their range of catalysis. 

Table 2.— Free diameters of apertures governing access to 
channels, assuming radius of oxygen = 1.35 A (6) 



Zeolite 

Chabazite ........ 

Edingtonite 

Erionite 

Faujasite 

Heulandlte 

Mordenite 

Offretite 

Paulingite 

Phillipsite 

Stilbite 

Thomsonite 

Zeolite A 

Zeolite ZK-5 



Free 
Ring type dimensions, A 

8-ring 3.6 by 3.7 

. . do 3.5 by 3.9 

. . do 3.6 by 5.2 

12-ring 7.4 

8-ring 4.0 by 5.5 

10-ring 4.4 by 7.2 

12-ring 6.7 by 7.0 

8-ring 2.9 by 5.7 

12-ring 6.4 

8-rlng 3.6 by 5.2 

. . do 3.9 

..do 3.9 

..do 4.2 by 4.4 

. . do 2.8 by 4.8 

. .do 3.3 

10-ring 4.1 by 6.2 

8-ring 2.7 by 5.7 

. . do 2.6 by 3.9 

..do 4.1 

..do 3.9 

3.9 



A, Atomic 



B, Tetrahedral 



C, Crystal 






Figure 4.— Common depictions of zeolite structure: sodalite alpha cage. A, Ball and stick model showing atoms (silicon— filled 
circles, oxygen— open circles); B, solid tetrahedral model; C, polyhedral model. Solid lines connect the tetrahedrally coordinated 
cations in the structure. 



Si0 4 , AIO, 




□ O 



4R 



6R 



/ X / N 



D4R 
\ 








/ 



\ 



D6R 
/ 




A, ZK4 



Faujasite 



Figure 5.— Possible pathways for formation of faujasite and 
A-type zeolites. (Reprinted with permission from Silicon and 
Aluminum Ordering of Zeolites: Interpretation of Si-29 NMR Data for 
Faujasite and ZK4 (p. 247), by M. T. Melchior. Paper in Intrazeollte 
Chemistry, ed. by G. Stuky and F. Dwyer. V. 218. Copyright 1983, 
American Chemical Society.) 




However, the apertures can serve as molecular sieves for 
selective catalysis inside the cavities. Molecules that can 
enter the cavities and be catalyzed include oxygen, am- 
monia, hydrogen sulfide, methanol, sulfur dioxide, straight- 
chain hydrocarbons, and water. 

Miller (19) stated that because the apertures of the trun- 
cated octahedral polyhedral framework are large enough to 
permit virtually any molecule to enter the cavity, they do 
not function as molecular sieves. These large apertures, 
however, allow metal atoms to enter the cavity and sub- 
stitute for alkaline and alkaline-earth metals in the struc- 
ture. A wide range of molecules (including cyclopentane, 
benzene, fluorine compounds, high-molecular-weight 
hydrocarbons, alkyl aromatics) can enter the cavity to par- 
ticipate in reactions that take place on or near these 
catalytic metals (fig. 8). 

The dimensions in table 2 refer to theoretically ideal, 
hydrated zeolites. Complete dehydration can produce ir- 
reversible structural changes (11). Stacking faults can block 
channels or reduce the effective aperture size (11). En- 
trained impurities can make large differences in effective 
channel sizes, and ion-exchange replacement of cations with 
those of different ionic radii change the effective channel 
size (lJf,, 23). A good example of this is that zeolite A is sold 
as "3A" (effective 3- A channel size), "4A," or "5A." The 
aluminosilicate framework topology is identical for all three, 
but different cations effectively change channel size (1). 

B 





Figure 6.— Polyhedra orientation and zeolite structure. 4, 
Symmetrical configuration of epsilon cages in zeolite L; S, 
nonsymmetrical configuration similar to that observed in can- 
crinite, epsilon cages in erionite, and offretite. (Reprinted by per- 
mission from Zeolite Molecular Sieves (p. 113), by D. W. Breck. 
Copyright (©) 1974, John Wiley & Sons, Inc.) 

E | , . G 





Figure 7.— Channels in zeolites. A and S, faujasite, zeolite X, and zeolite Y; C, zeolite A; D, ZK-5; E, chabazite; F, gmelinite; G, 
erionite; H, levynite. (Reprinted by permission from Zeolite Molecular Sieves (pp. 62-64), by D. W. Breck. Copyright (©) 1974, John Wiley & Sons, 
Inc.) 





Figure 8.— Effect of aperture size on catalytic properties. A, 
4- A aperture diameter of zeolite A prevents large molecules 
from entering structural cavities; S, 9-A aperture diameter of 
zeolites X and Y permits large molecules to enter the structural 
cavities and undergo catalysis. 

Zeolite classification has evolved with increasing 
knowledge about the structure of zeolites. Earlier classifica- 
tion based on morphology had erionite, for example, 



grouped with other fibrous structures. Recent classifica- 
tions are based on the framework topology. Breck (22) 
grouped zeolites by common SBU's into seven categories 
(table 3). No zeolite is typical of all other zeolites within a 
group because of slight differences in chemistry and 
crystallography. 

Table 3.— Zeolite structural groups 



Group 



Secondary building unit (SBU) Zeolites within group 

Single 4-ring (S4R) Analcime, phillipsite. 

Single 6-ring (S6R) Erionite, offretite. 

Double 4-ring (D4R) Zeolites A, P. 

Double 6-ring (D6R) Faujasite, chabazite. 

Complex 4-1, T s O,„ Natrolite, thomsonite. 

Complex 5-1, T 8 0, e Mordenite, epistilbite. 

Complex 4-4-1, T, O 20 Heulandite, stilbite. 



FORMATION PROCESSES AND GEOLOGIC OCCURRENCE 



According to Hay [24), most zeolite occurrences in 
nature are assignable to six types of geologic environments 
or hydrological systems: (1) saline, alkaline lakes, (2) saline, 
alkaline soils and land surfaces, (3) sea-floor sediments, (4) 
percolating water in an open hydrologic system, (5) 
hydrothermal alteration, and (6) burial diagenesis or 
metamorphism. Iijima (25) lists two additional geological oc- 
currences; magmatic zeolites and zeolites forming in impact 
craters. The largest and most commercially valuable 
deposits within the United States are those of categories 1, 
2 and 4 (U). 

Zeolites commonly form by direct precipitation from 
pore fluids as in magmatic and some hydrothermal occur- 
rences (25) or by the alteration of volcanic glasses or poorly 
crystalline silicate minerals (26). The most common parent 
materials are volcanic glass, clays, montmorillonite, 
plagioclase, nepheline, biogenic silica, and quartz. Under the 
proper conditions, one zeolite species may replace another 
species. Temperature, pressure, chemical activity of ionic 
species, and the partial pressure of water all affect which 
species of zeolite forms (26). Temperatures of formation can 
range from ambient to 700° C, and pressures can range 
from 1 to 1,000 atm (11, 24). 

Figure 9 from Hay (24) depicts the modes of occur- 
rence of natural zeolites and the approximate depths at 
which these zones may be found. 

Zone A in Figure 9 is characterized by nonanalcimic, 
alkali-rich zeolites, zone B by analcime or heulandite, and 
zone C by K-feldspar in 9 A, B, C, and D and by albite with 
or without laumontite in 9 E and F. 



SALINE, ALKALINE LAKES 

Saline, alkaline lake environments are associated with 
two types of tectonic settings in arid and semiarid regions: 
block-faulted terrains and trough valleys associated with 
continental rifting (26-27). These settings establish a closed 
basin system and control the transport of clastic material 
beyond the basin edges (27). Both are essential for control- 
ling lake chemistry (24, 26-27). 



The restricted lakes in which zeolites form are alkaline, 
with pH values of 9.5 (26). Analcime, clinoptilolite, phillip- 
site, erionite, chabazite, and mordenite commonly form, 
replacing volcanic glass, biogenic silica, poorly crystallized 
clay, montmorillonite, plagioclase, nepheline, and quartz 
(20, 26). These deposits are zoned laterally, as in figure - 9 A, 
representing the changing chemistry of the water as 
evaporation and precipitation of minerals proceed (27). 

The zonation of the mineral beds in the various saline, 
alkaline lakes indicates the sequence of the zeolite 
diagenesis (24): 

1. Glass ► alkalic, silicic zeolites. 

2. Alkalic, silicic zeolites ► analcime. 

K-feldspar. 

► K-feldspar. 



3. Analcime * 

4. Alkalic, silicic zeolites 

Some controlling chemical parameters during the glass- 
to-zeolite reactions are the cation ratios, silicon-aluminum 
ratios, and the activity of the water. Dissolution of the glass 
is controlled by the salinity and pH (24). These reactions can 
be rapid, with vitreous tuffs altering to zeolites in less than 
1,000 yr (26). Ancient Lake Tecopa, shown in figure JO, is 
typical of the saline, alkaline lake environment. 

SALINE, ALKALINE SOILS 

Climate is the controlling factor in the formation of 
zeolites in saline, alkaline soils. These deposits form in arid 
and semiarid regions where evaporation causes sodium 
carbonate-bicarbonate to concentrate in the surface soils 
(26). Rainwater percolating through the soils dissolves the 
sodium carbonate-bicarbonate and increases in pH, allowing 
it to alter glasses and aluminosilicates present in the soil 
(26). The water table is the probable limit of zeolitic altera- 
tion processes. Zeolite deposits of this type can approach 18 
min thickness and contain 15% to 40% zeolites. Analcime is 
the most common zeolite forming in these formations (24). 
Minor amounts of phillipsite, natrolite, and chabazite also 
occur. Deposits of this nature have been observed in the 
Olduvai Gorge, Tanzania (28), and other sites in Africa and 
in the Western United States (26). 








A, Saline, alkaline lake deposits 

Land surface 
Water table V 



X 



t ♦ ♦ ♦♦♦♦♦♦♦♦♦ 

♦ ♦♦ ♦♦♦♦♦♦♦♦♦ 
♦ ♦♦• ♦ ♦♦♦♦♦♦♦♦ 



B, Saline, alkaline soils 



100 m 



Tl-10 



m 



KEY 
♦ * J Fresh glass 
Altered glass 



A,B,C Zones 



A, 

minor 

B + C (?) 



* ♦ «■ * 

♦ ••..♦ 



♦ ♦ ♦ r ♦ 



C, Deep sea sediments 



1,000 m 




-r- Su 




10-20 
m 



Silicic tephra 



* ♦ * ♦ Water table 



Basanite tephra 



D, Open hydrologic system 




500 m 



E, Hydrothermal alteration 



♦ ♦ ♦ ♦ 



-5,000 m 



.10,000 m 



F, Burial diagenesis 



Figure 9.— Patterns of authigenic zeolites and feldspars. {A, B, and C reprinted by permission from Geology of Zeolites in Sedimentary 
Rocks (p. 55), by R. L. Hay. Ch. in Mineralogy and Geology of Natural Zeolites, ed. by F. A. Mumpton. Copyright 1977, Mineralogical Society of America. 
D, E, and F, reprinted by permission from Geologic Occurrences of Zeolites (p. 137), by R. L. Hay. Ch. in Natural Zeolites: Occurrence, Properties, Use, 
ed. by L. B. Sand and F. A. Mumpton. Copyright 1978, Pergamon Press.) 



CALIFORNIA 




OPEN HYDROLOGIC SYSTEMS 



Zeolites in open systems form from the percolation of 
ground water through porous pyroclastic materials rich in 
reactive glass (25). The pH and dissolved solid content of the 
ground water increase as it reacts with the vitric ash, until 
zeolites are precipitated (24). Movement of the ground 
water downward through the system results in a vertical 
zonation of water composition and authigenic minerals, in- 
cluding zeolites (24). Topically a silicic tephra may contain 
an upper zone of fresh glass, montmorillonite, and opal. The 
next bed can contain up to 90% clinoptilolite, and the 
underlying zone may contain analcime, potassium, feldspar, 
and quartz (26). Alteration of the glass can occur rapidly 
because of the high pH (approximately 9.5) resulting from 
hydrolysis (24). 

An example of an open-system deposit is the basanite 
vitric tuffs of Koko Crater in Hawaii (24). These tuffs con- 
sist of 1.5 to 12 m of fresh glass, opal, and montmorillonite, 
The next lower zone consists of 1.5 to 10 m of palagonite 
tuffs with phillipsite and chabazite. The lowest zone is 8 m 
thick and predominantly analcime (24). 



HYDROTHERMAL SYSTEMS 



Figure 10.— Ancient Lake Tecopa near Shoshone, CA, show- 
ing diagenetic fades. (From Distribution and Genesis of Authigenic 
Silicate Minerals in Tuffs of Pllestocene Lake Tecopa, Inyo County, 
California (p. 20), by R. A. Sheppard and A. J. Gude 3d. U.S. Geol. Surv. 
Prof. Paper 597, 1968.) 



MARINE SEDIMENTS 

Zeolites form in marine sediments under low 
temperatures and moderate pH conditions (pH values of 7 
to 8) (25). Zeolites in the slow sedimentation regions of the 
Pacific and Indian Oceans occur in post-Miocene brown 
clays, vitric siliceous and calcareous oozes, and basaltic 
volcanic sediments. Phillipsite is the dominant species (25). 
Those in the rapid sedimentation regions of the Atlantic and 
Pacific margin are in calcareous sediments and terrigenous 
clays of Paleogene and Cretaceous ages. Clinoptilolite is the 
dominant species here. Analcime, chabazite, erionite, 
laumontite, gmelinite, natrolite, and thomsonite also may be 
present in these marine deposits (25). 

Zeolites in sea-floor sediments form by the reaction of 
glasses with pore water (26). Except for the silica content, 
which ranges from 4 to 65 ppm, sediment pore water is 
similar to seawater in composition (24). Phillipsite is 
associated with the low silica concentrations (less than 20 
ppm) typical of basaltic tephras, and clinoptilolite is 
associated with the high silica concentrations (20-40 ppm) of 
siliceous tephras (24). Clinoptilolite occurring with phillip- 
site probably forms when basaltic glass reacts with silica- 
rich pore waters (25). 

Boles and Wise (29) suggest that since marine clinop- 
tilolite occurs as euhedral crystals, their formation must 
have been from pore water rather than as a direct replace- 
ment of earlier phases. In spite of this, they think that 
because phillipsite is found in the younger sediments it is 
likely that the clinoptilolite in the older sediments replaced 
it and that time was an important factor. 



Zeolites precipitate in hydrothermal systems from 
alkaline to weakly acidic hot water (25). The assemblages 
observed are controlled by temperature, host rock composi- 
tion, host rock permeability, and geothermal fluid composi- 
tion (25). Clinoptilolite and mordenite occur in the 
shallowest and coolest zones. Analcime or heulandite and 
laumontite or wairakite, the less hydrated forms, occur in 
the deeper and hotter zones (24, 26). Submarine hydrother- 
mal activity may be responsible for the formation of some 
zeolites in deep-sea sediments (SO). 

Examples of hydrothermal zeolite deposits are found in 
Yellowstone Park, WY; Wairakei, New Zealand; and 
Onikobi, Japan (26). At the Onikobi site in Japan, there are 
four zones: a mordenite zone, a laumontite zone, a 
laumontite-wairakite zone, and a wairakite zone (25). 

BURIAL DIAGENETIC SYSTEMS 

Zeolites associated with burial diagenesis occur in thick 
volcanoclastic sediments that were metamorphosed at in- 
creased temperatures (26). Two reaction sequences are 
recognized, the alkali zeolite reaction series and the calcic 
zeolite reaction series (25). The alkali series includes the 
zeolites clinoptilolite, mordenite, and analcime. The calcic 
series includes clinoptilolite, heulandite, and laumonitite 
(25). Burial diagenetic sequences are typified by vertical 
zonation (26). This mineralogical zonation is related to in- 
creasing burial depth (25). Iijima (30) recognized five zones 
based on the mineralogy of marine sediments subjected to 
burial diagenesis (fig. 11). Temperatures increase with 
depth, as in the Green Tuff region in Japan where 
temperatures were 41° to 49° C at the top of the zeolite se- 
quence and 120° to 124° C at the base of the sequence (26). 
More hydrated zeolites occur at shallower depths (24). The 
upper zones of burial diagenetic sequences are min- 
eralogically similar to open hydrologic systems. The major 
distinction is the gradual change between zones in the burial 
diagenetic sequence, compared with the sharp contacts of 
the open system deposit (26). 



Mineral species 


Zone 

I 


Zone 

II 


Zone III 


Zone 
IV 


a 


b 


"Silicic glass" 
Alkali clinoptilolite 
Clinoptilolite-Ca 
Alkali mordenite 
Mordenite-Ca 
Analcime 
Heulandite 
Laumontite 


■ ™ ■ 




■■ ■ 




■ 












K-Feldspar 

Albite from analcime 














Albitized plagioclase 

Opal-Ct 

Quartz 

Montmorillonite 

15A-14A mixed layer 


















m ^ 


2^m 


Chlorite 














Illite 

Prehnite 

Pumbellyite 






™ 



Figure 11.— Zonal distribution of zeolites and silicates in 
burial diagenesis. (Reprinted with permission from Geologic Occur- 
rences of Zeolite in Marine Environments (p. 179), by A. lijlma. Ch. in 
Natural Zeolites: Occurrence, Properties, Use, ed. by L. B. Sand and F. A. 
Mumpton. Copyright 1978, Pergamon Press.) 

An example of burial diagenetic deposits is the Green 
Tuff region of Japan (24). In the Niigata oilfield, the upper 



zone is 0.8 to 1.9 km thick, containing fresh glass. The next 
zone, 1.6 to 2.5 km thick, contains mordenite and clinop- 
tilolite. This is followed by an analcimic zone, 1 km thick, 
and an albitic zone, 0.7 km thick. Analcime is pseudomor- 
phic after mordenite and clinoptilolite. Temperatures range 
from 41° to 124° C. 



MAGMATIC SYSTEMS 

Zeolites crystallize during the late stages of formation 
of magmatic rocks (25). They commonly occur as fine 
crystals lining vugs in basic igneous rocks, although they 
also occur as interstitial grains and globules (6, 25). 
Crystalline zeolites form through interaction of fluids with 
the surrounding rock (6). Zeolite globules form when water- 
rich magma separates into two immiscible liquids (25). 

The zeolites that form under magmatic conditions are 
grouped by bulk composition. Aluminum-rich zeolites occur 
in basic igneous rocks with low silicon-to-aluminum ratios 
(6). Aluminum-poor zeolites occur in igneous rocks richer in 
silicon. Several zeolites that occur in igenous rocks are 
analcime, clinoptilolite, heulandite, mesolite, mordenite, 
natrolite, phillipsite, and stilbite (25). 



IMPACT CRATERS 

Iijima (25) reports zeolites occurring in the Nordlinger 
Ries impact crater in West Germany. In this occurrence, 
glass formed by the meteor impact altered to analcime and 
smectite. Microcavities in the deposits were filled by 
analcime, clinoptilolite, erionite, harmotone, and phillipsite. 



EXPLORATION 



Extensive exploration for natural zeolite deposits did 
not begin until the 1950's, when Union Carbide Corp., 
through its Linde Division, initiated a search for minable 
quantities of natural zeolites that might compete with their 
synthetic zeolites (SI). In describing the onset of this proj- 
ect, Mumpton (31) reports that mineralogical and mineral- 
commodity consultants employed by Union Carbide did not 
believe that minable deposits of zeolites would be found. 
They, like most researchers, believed that zeolites were 
mineralogical curiosities that filled vugs and fractures in ig- 
neous rocks (6). However, in the 5-yr effort, over 3,000 
samples were collected and approximately 200 new zeolite 
occurrences in volcanoclastic sedimentary rocks were 
discovered. 

As the initial search was going on, a thorough literature 
search revealed earlier discoveries of sedimentary deposits. 
Coombs (32), at the University of Ofago in New Zealand, 
described the widespread occurrence of laumontite, 
analcime, and heulandite in burial metamorphic sequences 
of pyroclastic sediments in that country and demonstrated 
the occurrence of substantial amounts of zeolites in 
"nonigneous" environments. Ames, Sands, and Goldich also 
described large occurrences of clinoptilolite at Hector, CA 
(7). This information prompted Union Carbide's Linde Divi- 
sion to intensify its investigation of zeolite occurrences in 
pyroclastic rocks. 

Samples from the Hector deposit were examined by 
standard X-ray diffraction and microscopic techniques (31). 
Mumpton reported that "several fine-grained, homoge- 



neous, lightweight samples from the Hector deposit, 
representing beds as thick as 12-15 ft. were found to contain 
as much as 80-85% clinoptilolite," and that "the Hector 
samples demonstrated conclusively that near- 
monomineralic deposits of natural zeolites could indeed be 
found in mineable quantities within the continental United 
States. They also demonstrated that the few reports in the 
earlier geological literature about the occurrence of zeolites 
in sedimentary rocks of volcanic origin were not flukes, but 
that many more such deposits were likely to be found in 
geologically similar environments throughout the western 
part of the country and probably throughout the world." 
(31). 

Early in 1958, a sample described as erionite from a 
million-short-ton, easily mined, sedimentary deposit was 
submitted to Linde (31). Laboratory examination indicated 
that the sample contained 85% to 90% erionite and that its 
chemical, adsorption, optical, and X-ray diffraction proper- 
ties closely matched those of Linde's synthetic analog, 
zeolite T. Later, the Kennedy Minerals Co., discoverer of 
the deposit, revealed that the deposit was located in a broad 
basin of lacustrine and fluviatile sediments about 2 miles 
west of Rome, OR. Here, the Owyhee River had cut through 
plateau basalts to expose several hundred feet of 
volcanogenic sedimentary rocks (31). 

The Linde exploration program remained active until 
1961. As it tapered off, Shell Development Co. started a 
modest exploratory effort led by H. D. Curry (31). Regnier 
(S3), at Columbia University, described beds of high-grade 



erionite, clinoptilolite, and an unidentified zeolite (later iden- 
tified as phillipsite) in altered Cenozoic strata in Pine Valley, 
NV. Deffeyes (84) reported that these same zeolites had 
been recognized in near-monomineralic beds of altered tuff 
in Jersey and Reese River Valleys, NV, and that zeolites 
seemed to be common in the rocks of the Rocky Mountain 
west. Zeolites had not been previously recognized in these 
rocks because of their extremely fine-grained nature (1 to 5 
fim) and the tendency of the field geologist and 
stratigrapher to limit their laboratory studies of these 
materials to microscopic examinations (31). Once X-ray dif- 
fraction techniques were employed, the presence of zeolites 
became much more widely recognized in Cenozoic 
volcanogenic sediments. 

Table 4 from Mumpton (81) lists the discovery dates of 
32 prominent natural zeolite deposits. Nearly 90% were 
discovered between 1957 and 1962. 

Mumpton's own summary of his paper is given in its en- 
tirety below because it contains so much of the story of 
natural zeolites in the United States (81): 

In the five-year period beginning in January 1958, and 
ending in December 1962, more than 8000 samples were col- 
lected and approximately 200 new occurrences of zeolite 
minerals in volcanoclastic sedimentary rocks were 
discovered as a result of the exploration efforts of Union Car- 
bide Corp. Nearly 100 other occurrences of these minerals in 



the United States became known at the time as a result of 
other discoveries by university geologists (e.g., 
L. B. Sand, F. B. Van Houten, R. L. Hay, K. S. Deffeyes, 
Jerome Regnier) and by employees of the U.S. Geological 
Survey (e.g., R. L. Smith, R. E. Wilcox, H. A. Tourtelot, 
Theodore Botinelly, A. D. Weeks, D. L. Peck, R. C. Erd), and 
from the literature searchings of these individuals and 
Linde's research personnel. 

Much of Carbide's exploration effort was guided em- 
pirically by the newly developed knowledge that zeolites were 
abundant in (1) flat-lying, light colored tuffaceous beds of 
Cenozoic age which commonly crop out as erosion-resistant 
ledge-formers in association with unaltered volcanic ash, 
bentonites, and other typical lacustrine formations; and (2) 
light-colored, altered ash-flow tuffs in Tertiary pyroclastic 
bodies commonly associated with perlites and welded tuffs. 

Aerial reconnaissance of Tertiary basins, the field test 
developed by Linde, and the routine use of X-ray diffraction 
for the identification of exploration samples proved to be ef- 
fective. The direct communication between the field geologists 
and the chemists and mineralogist at Linde and the short 
turnaround time for field samples gave rise to technical 
assistance that could not be provided by the Nuclear Divi- 
sion, which itself had no previous experience with zeolites or 
with non-metallic commodities in general. 



Table 4.— Discovery and/or Initial commercial Investigation of prominent zeolite deposits in the United States 



Occurrence 



Zeolite 



Year 



Alabama: Campbell Clinoptilolite 1965 

Arizona: 

Bowie Chabazite 1959 

Clinoptilolite 1959 

Erionite 1959 



Dripping Springs Valley Chabazite . . . 
(Christmas). 

Horsehoe Dam Clinoptilolite 

Union Pass Mordenite . . . 

Wellton Clinoptilolite 

Wikieup Analcime 

Clinoptilolite 



1961 

1961 
196C 
1962 
1928 
1958 



Phillipsite 1959 

California: 

Barstow (Rainbow Basin) Analcime 1959 

Clinoptilolite 1959 

Phillipsite 1959 

Death Valley Junction Clinoptilolite 1973 

Hector do 1957 

Shoeshone (Lake Tecopa) Analcime 1959 

Clinoptilolite 1959 

Erionite 1959 

Phillipsite 1959 

Colorado: Creede Clinoptilolite (1970 

(1977 



Idaho: Castle Creek do 

Nevada: 

Ash Meadows do 

Eastgate Erionite . . 

Mordenite 



1961 

1960 
1958 
1962 



Fish Creek Mountains Clinoptilolite (1957 

11958 

Hungry Valley Chabazite 

Clinoptilolite 



.... 1960 
.... 1960 

Jersey Valley Erionite (1957 

(1958 



Occurrence 



Zeolite 



Year 



Nevada— Continued 

Lovelock Clinoptilolite 

Ferrierlte 
Mordenite . . . 

Pine Valley Erionite 



.... 1959 
.... 1966 
.... 1959 
.... 1957 
1958 

Pine Valley(Clay Mounds) Phillipsite 1957 

1958 

Reese River Valley Chabazite 1958 

1959 

Clinoptilolite 1958 

Erionite 1957 

New Mexico: Buckhorn Clinoptilolite 1962 

Oregon: 

Durkee Chabazite 1970 

Clinoptilolite 1970 

Erionite 1958 

Harney Lake (Narrows) Clinoptilolite 1959 

Erionite 1959 

Mordenite 1959 

Rome Erionite 1957 

Mordenite 1962 

Sheaville Clinoptilolite 1958 

South Dakota: Pine Ridge . . do (1958 

Reservation. (1961 

Texas: 

Marfa do 1962 

Tilden .do 1964 

f1972 

Utah: Cove Fort do J 1979 

1 1980 
Wyoming: 

Beaver Divide Chabazite 1966 

Clinoptilolite 1957 

Erionite 1958 

Fort LeClede(Washakie Basin) . . Clinoptilolite (1973 

(1977 



10 



Where in 1957, zeolites were thought to exist primarily 
as late-stage hydrothermal products in amygdaloidal 
basalts, by 1962 they were being recognized as common con- 
stituents of altered pyroclastic material in a wide variety of 
sedimentary and volcanic environments. During this 
period, the heretofore rare zeolite erionite was found to occur 
in large, mineable deposits, and the widespread distribution 
of clinoptilolite in such rocks was firmly established. The 
first verified occurrences of chabazite and mordenite in 
sedimentary rocks of the United States were also discovered 



during the Carbide program. The flat-lying, near-surface, 
and near-monomineralic nature of the deposits suggested 
that many would eventually be mined and that the so-called 
sedimentary zeolites would one day become an accepted in- 
dustrial mineral commodity. 

The sudden interest in zeolites was further emphasized 
by the fact that of the more than 400 published reports that 
describe zeolites in sedimentary rocks throughout the 
world, more than 75% were published in the 1950's and 
1960's (M). 



SYNTHESIS (11) 



The synthesis of zeolites was first reported by St. Clair 
Deville in 1862. By heating aqueous solutions of potassium 
silicate and sodium aluminate in a glass tube at 170° C, 
Deville produced the zeolite levynite. In 1882, De Schulten 
reported the synthesis of analcime. However, data are not 
available to substantiate these and other experiments, and 
much of the early work cannot be duplicated in the 
laboratory. The first substantiated synthesis of zeolites was 
not performed until the 1940's when X-ray diffraction was 
used to identify phases. Prior to this time, light microscopy 
was used for phase identification, and the fine-grained 
nature of synthesized zeolites made identification difficult. 
A list of zeolites synthesized prior to 1936 is presented in 
table 5. 

Early attempts at zeolite synthesis sought to duplicate 
the conditions under which zeolites were thought to 
crystallize in basaltic rocks. In 1959, R. M. Milton and 
associates at Union Carbide Corp. suggested a new ap- 
proach, which allowed the synthesis of zeolites at low 
temperatures. Their method used highly reactive ingre- 
dients in a closed system at low temperatures, often below 
the boiling point of water. This made the large-scale produc- 
tion of synthetic zeolites feasible (1). 

Breck reports the general conditions used in zeolite syn- 
thesis: 

1. Reactive starting materials such as freshly copre- 
cipitated gels or amorphous solids. 



2. Relatively high pH introduced in the form of an alkali 
metal hydroxide or other strong base. 

3. Low-temperature hydrothermal conditions with con- 
current low autogeneous pressure at saturated water vapor 
pressure. 

4. A high degree of supersaturation of the components 
of the gel, leading to the nucleation of a large number of 
crystals. 

The following are simultaneous reactions that occur during 
the synthesis process (13): 

Precipitation of a gel phase, 

Dissolution of the gel, 

Nucleation of zeolite(s), 

Continued crystallization and crystal growth of the 
zeolite(s), 

Dissolution of the initial metastable phase(s), 

Nucleation of a more "stable" metastable phase or 
phases, 

Continued crystallization and crystal growth of the new 
crystalline phase(s) while the initial crystals are dissolving, 

Dissolution of the metastable phase(s), 

Nucleation of the equilibrium phase(s), 

Crystallization and crystal growth of the final 
crystalline phases. 



Table 5. — Selected summary of unsubstantiated zeolite synthesis (17) 



Date 

1862 

1880 

1882 

1883 

1885 

1887 

1890 

1894 

1896 

1906 

1911 

1916 

1918 

1927 

1929 

1936 

1 Identified as kieratite. 

2 Not indicated. 



Zeolite 

Levynite 

Analcime 

. . do 

. . do 

..do 

..do 

Chabazite 

Natrolite 

Thomsonite 

Analcime 

Natrolite 

K faujasite 1 

Analcime 

..do 

Mordenite 

Natrolite 

Analcime 



Hydrothermal method 

K silicate + Na aluminate 

Na silicate + AljOj glass 

Na silicate + Na aluminate . . . 
SiO;, NaOH solution, Al 2 3 

Conversion ot chabazite 

Kaolin + Na silicate 

Recrystallization 

Anorthite 

Muscovite + NaOH 

Nepheline + Na 2 Co 3 + H 2 . . . 

Na : Co 3 + Al 2 3 + SiO, 

K,0, Al ; 3 , SiO : 

Adularia + NaAI0 2 

Na 2 0, Al 2 3 , Si0 2 

Feldspar + Carbonates 

Paragonite + NaOH 

Na silicate + Na aluminate . . . 



Temperature of 
formation, °C 



Investigator 



170 
180 
180 
400 

200 
200-220 
150-170 
174-177 

200 

200 
90 

350 

280 

300 

(2) 
400 
282 



St. Claire Deville. 
A. de Schulten. 

Do. 
C. Fried, 
E. Sarasin. 
J. Lemberg. 

Do. 
C. Doelter. 
St. J. Thugutt. 
C. Friedel. 
C. Doelter. 

E. Baur. 

E. A. Stephenson. 
W. J. Muller, 
J. Konigsberger. 
R. J. Leonard. 

E. Gruner. 

F. G. Straub. 



11 



Because of the multitude of reactions, time is an important 
factor determining phase stability and crystal ordering. 
Breck states "that a mineral zeolite which has existed over 
long periods of geological time and a synthetic zeolite with a 
related structure but which was synthesized rapidly in the 
laboratory may exhibit differences in properties due to the 
ordering that may occur in the mineral as opposed to the 
lack of ordering the synthetic structure shows." Many of the 
synthetic zeolites that are not structurally related to any 
natural zeolite may be nonequilibrium phases. Metastable 
synthetic structures may also explain why many natural 
zeolites do not have synthetic counterparts despite the syn- 
thesis of over 100 different zeolite types. 

Several processes are used to synthesize zeolites. In one 
process, kaolin is used as the source of the silica and alumina 
required in the reaction because of its higher reactivity. 
Equations 1 and 2 (modified to omit unreacted water) give 
the reaction sequence. 



2Al 2 Si 2 B (OH) 4 
(kaolin) 



500°-600° 



Sw 2Al 2 Si 2 7 +'4H 2 (1) 
(metakaolin) 



6Al 2 Si 2 7 +12NaOH + 21H 2 
(metakaolin) 

100° C 



Na 12 (A10 2 ) 12 (Si0 2 ) 12 |-|27H 2 
(zeolite A) 



(2) 



A material balance of equation 2 shows that for each 
short ton of zeolite A produced 1,217 lb of metakaolin are 
needed, 438 lb of sodium hydroxide, and 345 lb of reaction 
water. Other water is needed to provide the necessary 
aqueous environment but does not react. For equation 1, 
1,414 lb of kaolin are necessary to produce 1,217 lb of 
metakaolin. 

Zeolites can also be synthesized from aluminosilicate 
gels of the alkali earths, either potassium, sodium, lithium, 
rubidium, and/or cesium. In this process, the starting gel is 
depolymerized by the hydroxyl ions present in the starting 
mixture, and zeolite crystallites form (U). Magee and 
Blazek (35) discuss the synthesis of zeolite Y using silica gels 
as a basic ingredient. The procedure they report is as 
follows: 

1. Place 2,090 g H 2 in a resin kettle. 

2. Dissolve 162.2 g NaOH in the water. 

3. Add 425 g of sodium aluminate, containing 24 wt % 
A1 2 3 , 20 wt % Na 2 0, and 56 wt % H 2 0. 

4. Cool to 100° F, and then add 570 g of calcined silica. 

5. Cold-age at 100° F for 24 h with mild agitation. 



6. Increase temperature to 212° F, and age for 24 h 
with mild agitation. 

7. Quench with water, and filter to remove the solids 
from the mother liquor. 

8. Wash with hot water, and dry in forced-air oven at 
250° F. 

This process yields 464 g of sodium-Y zeolite 
(Na 2 OAl 2 Cv5Si0 2 -nH 2 0). The silicon-aluminum ratio is 
2.5 in this process; it was 1 for zeolite A. Small differences 
in molar ratios can produce entirely different zeolites under 
identical conditions. The form of the starting materials can 
also affect mineral stability. Type A, X, and Y synthetic 
zeolites are examples of synthesis from a gel. Type A 
as synthesized or as modified, was the first zeolite to 
become available in sufficient quantities to permit extensive 
laboratory study. Types X and Y had sufficient pore size, 
cage volume, and accessibility to selectively permit entry 
and controlled reaction of the larger molecules common to 
the petrochemical industry. 

The main difference between the X and Y catalysts is 
the silicon-aluminum ratio. The silicon-aluminum ratio for 
zeolite X ranges from 1 to 1.5 while that for zeolite Y ranges 
from 1.5 to 3. The diameter of the large cage of both is 13 
A, but the unit cell of X is slightly larger, 25.02 to 24.86 A, 
while that of Y is 24.85 to 24.61 A. Table 6 lists some 
of the zeolites that can be synthesized from the 
Na^-A^CvSKVH^O system and the reactants used. 
Most of these zeolites have a sodium oxide-to-alumina ratio 
of 1. 

Quaternary ammonium hydroxides, such as tetramethyl 
ammonium ((CH 3 ) 4 *NOH), can also be used as a source of 
cations in zeolite synthesis. The ammonium compounds act 
as templates to control the structure of the zeolite (10). This 
process led to the synthesis of some new and technically im- 
portant zeolites such as Mobil's ZSM-5 (11, 36). The basic 
process consists of reacting an aluminosilicate gel contain- 
ing sodium and tetrapropyl ammonium cations. Varying 
silica-alumina ratios are obtained by adjusting the silica-to- 
alumina ratio in the starting reaction mixture. Such ad- 
justments do not alter the framework topology. 

The recurring arrangement of tetraheda into five- 
membered rings is evident in figure 112, which shows both a 
one-dimensional and a two-dimensional view of the struc- 
ture of ZSM-5. 

The channel system of ZSM-5 "consists of a 3D intersec- 
tion of a straight channel with elliptical cross-section 
.51x.57nm along the orthorhombic 6-axis (1.98nm) and a 
zig-zag channel along [101] and [107] directions (a = 2.01; c 
= 1.34nm). Four channels meet at each intersection" (fig. 
13) (37). To avoid possible confusion caused by the perspec- 
tive, the dimensions of the channels along the b-axis are 



Table 6.— Synthetic zeolites: Na 2 0>AI 2 3 *Si0 2 >H,0 system (71) 





Zeolite 
type 




Typ 


cal reactant compos 


tion, 




Temp, 




Zeolite composition, 










mol 


per mol Al 2 3 








mol 


per mol Al 2 3 






Na 2 






Si0 2 




H 2 




Na 2 




Si0 2 


H 2 


A 




2 






2 




35 


20-175 


1 




2 


4.5 


R 




3.2 






4 




260 


100 


1 




3.5 


5.7 


R 




2.4 






6 




80 


80-120 


1 




4.6-5.9 


6 


X 




3.6 






3 




144 


20-120 


1 




2.0-3.0 


6 


Y 




8 






20 




320 


20-175 


1 




3.0-6.0 


9 


HS.. 




2.8 
4 






3.0 
2 




34 


100 
100 


1.16 
2.0 




2.1 
2.1 


2.8 
2.5 



NOTE.— Reactants for zeolites A, R, S, X, and Y are sodium aluminate, sodium silicate, sodium hydroxide, and colloidal silica; reactant for zeolite 
HS is silica gel. 



12 





Figure 12.— Structure of ZSM-5. A, Two-dimensional view; S, 
three-dimensional view. 



Elliptical 10-ring of 
straight channel 




Figure 13.— Channel system of ZSM-5. (Reprinted by permis- 
sion from Review of New Crystal Structures and Mineralogy of Zeolites 
and Related Materials (p. 196), by J. V. Smith. Paper in Proceedings of 
the Fifth International Conference on Zeolites, ed. by L. V. C. Rees. 
Copyright (©) 1980, John Wiley & Sons.) 

shown at the top of the figure. The straight channels along 
the b-axis are seen from above in figure 12. 

The synthetic zeolite ZSM-5 has an almost infinitely 
variable silica-alumina ratio (fig. 14 ). Smith (37) reports the 



V//A Region of zeolite 
stability 




Al (RELATIVE TO Si ATOMS), pet 

Figure 14.— Aluminum atom concentration and SiOz'AbOi 
ratio for various zeolites. (Reprinted by permission from The Active 
Site of Acidic Aluminoslllcate Catalysts (p. 590), by W. 0. Haag, R. M. 
Lago, and P. B. Welsz. Nature, v. 309. Copyright (?) 1984, Macmillan 
Journals Limited.) 



formula of the sodium form as Na^AL^Si^C^ • 16H 2 0, 
with n less than 27 but typically equal to 3. "Whereas a 
structure with tetrahedral Al and corresponding exchange 
sites shows ion-exchange and reversible dehydration ex- 
pected of an ideal zeolite, the Al-free structure does not 
show ion-exchange and is hydrophobic and organophilic: 
thus silicalite is a molecular sieve but not a zeolite." 

Aluminum atoms are no longer structure- determining 
constituents; that is, they no longer contribute a major por- 
tion of the framework structure (38). The average distance 
between aluminum sites is equivalent to several atomic 
diameters, and their distribution should approach ran- 
domness. 



CHEMICAL AND PHYSICAL MODIFICATION 



DEHYDRATION AND REHYDRATION 

The dehydration properties of zeolites are important 
because they influence the adsorption characteristics of the 
zeolite (11). Dehydration can cause distortion of crystalline 
structure (39). This influences the volume and shape of the 
micropores (11). Zeolites that retain structural stability dur- 
ing dehydration have micropores that fill and empty rever- 
sibly so that adsorption is a matter of pore filling, and the 
surface area concepts and calculations common to other 
solid absorbents do not apply (11). Dehydration can also af- 
fect the charge distribution within the zeolite structure. 
This occurs when cations coordinated with water migrate to 
different crystallographic sites upon dehydration (11). Table 
7 summarizes the effects of dehydration on some zeolites. A 
wide variety of temperatures at which structural changes 



occur and an even greater variety of temperatures at which 
there are steps in the weight-loss curves of zeolites are 
demonstrated by thermogravimetric analysis. 



STRUCTURAL HYDROXYL GROUPS 

Structural hydroxyl groups within the zeolite structure 
are desirable for catalytic processes because they provide 
active sites for hydrocarbon conversions and other catalytic 
reactions (39). Hydroxyl groups can be introduced by two 
techniques, cation hydrolysis and deammoniation of NH 4 - 
exchanged zeolites. Cation hydrolysis can be induced by nor- 
mal dehydration, where a water molecule is dissociated, the 
hydroxyl is lost, and the hydrogen ion is fixed to structural 
oxygen atoms. Treatment of the zeolite with a strong acid 






13 



Table 7.— Summary of zeolite dehydration behavior 



Zeolite 



Weight loss 1 
Type 2 



Structural changes 



Remarks 



Group 1: 

Analcime . . 

Laumontite 

Phillipsite . 
Group 2: 

Erionite . . . 



Sodalite Hydrate 
Group 3: Zeolite A . . 
Group 4: 

Chabazite 

Faujasite 

Gmelinite 

Zeolite L 

Zeolite X 

Zeolite Y 

Group 5: 

Edingtonite 

Natrolite 



Thomsonite 

Group 6: Mordenite 
Group 7: 

Clinoptilolite .... 

Heulandite 

Stilbite 



3 8.7 

15.0 
•18.0 

14.8 
15.0 
22.5 

23.0 
25.0 
20.0 
16.7 
26.2 
26.0 

13.1 
9.7 

15.0 
16.0 

14.0 
17.0 
17.0 



None up to 700° 

Structure change at 500° . . 
New structure at 160 to 200° 



Stable to 750° 
Stable at 900° 
Stable to 700° 



No change at 700° 

Stable to 475° 

Structure change at 300° 

No change at 800° 

No change at 700° 

No change at 760° 



Recrystallizes to feldspar at 500°. 
New structure at 565°; amorphous at 

785°. 
Structure collapse at 520°. 
No change at 800 ° 

No change at 750° 

Heulandite "B" at 250 ° 

Change at 120°; collapse at 400°. 



None. 

Rehydrates below 200°. 

Rehydrate if not heated 250°. 

Stable to H,0 at 375°. 

High temperature from reported. 

B-cristobalite at 800°. 

Stability varies with cation. 
None. 

Do. 

Do. 
Stability varies with cation. 

Do. 

None. 

Rehydrates to 785°; nepheline at 

970° to 1,010°. 
No rehydration above 370°. 
None. 

Do. 
Structure collapse at 360°. 
Structure shrinks along b. 



' Based on original weight. Determined using thermogravimetric analysis. ' 100°, 250°, 400° 

2 C— continuous weight loss with increasing temperature; S— stepwise weight loss with increasing temperature. "240°. 

3 At 400°. '150°, 300°. 
'200°, 370°, 500°. "MOO , 250°. 
5 130°. "100°, 200°. 
•At 300°. 



will also produce hydrolysis through hydrogen exchange 
with available cations (11). Acid treatment, however, can 
cause structural degradation, especially in aluminum-rich 
zeolites. Deammoniation can also be used to create struc- 
tural hydroxyl groups (14). The ammonium ion is thermally 
decomposed, ammonia is evolved, and the hydrogen ion is 
fixed in the structure (11). By controlling the amount of 
deammoniation occurring, the number of structural hy- 
droxyl groups formed can be regulated (11). 



SORPTION AND DIFFUSION 
Molecular Sieving 

Molecular sieving refers to the selective adsorption of 
cations within a sorbent, based on physical dimensions and 
charge distribution (6). Molecular sieving depends on the 
characteristics of both the sorbent and the material being 
adsorbed. Zeolites are well suited for molecular sieving 
because they possess an open, yet uniform crystalline 
framework that has a narrow pore size distribution (39). 
This contrasts with other types of molecular sieves, such as 
silica gels or activated carbon, which have a wider range of 
pore size distributions (fig. 15). This property makes zeolites 
more size selective than other molecular sieves (11). 

Molecular sieving can be affected by heating and 
dehydration of the zeolite (11). Heating results in distortion 
of the crystalline framework and in aperture enlargement 
due to the increased vibrational amplitude of the structural 
oxygen as temperature increases (11). Dehydration results 



100r 



1 


KEY 




— Dehydrated zeolite 




— Typical silica gel 




— Activated carbon 




/ \ 



10 



100 



1,000 



5,000 



DIAMETER, A 



Figure 15.— Distribution of pore sizes in microporous ad- 
sorbents. (Reprinted by permission from Zeolite Molecular Sieves (p. 
4), by D. W. Brack. Copyright (©) 1974, John Wiley & Sons, Inc.) 

in cation displacement and subsequent changes in the 
charge distribution within the zeolite structure (11). 
Dehydrated zeolites selectively adsorb polar molecules, such 
as H 2 0, C0 2 , and H 2 S (39). This selectivity results from the 
unusual charge distributions within the structure due to the 
positions of cations, hydroxyls, and the substitution of 
aluminum for silicon in the framework structure (39). 

The size, shape, and charge of the adsorbed phase also 
influence molecular sieving (11). For instance, rubidium 
with a radius of 1.48 A is extensively exchanged by 
analcime, while sodium with a radius of 1.65 A is not ex- 
changed (11). Similar effects are observed with organics 
because of differences in their sizes and shapes (fig. 16). 



14 






Figure 16.— Kinetic diameters of some simple molecules. A, 
Methane and ethane; 6, propane; C, isobutane. (Reprinted by per- 
mission from Zeolite Molecular Sieves (p. 639), by D. W. Breck. 
Copyright (<§ ) 1974, John Wiley & Sons, Inc.) 

Polar molecules with their uneven charge distributions are 
selectively adsorbed by some zeolites (39). 

Diffusion 

Diffusion is the migration of sorbate within the crystal. 
It affects selective adsorption, desiccation, molecular siev- 
ing, and catalysis (6). Diffusion in zeolites is very com- 
plicated (40). Diffusivity, which is affected by molecule size 
and shape and framework topology, has proven to be ex- 
tremely difficult to predict (6, 40). Maurer (41) notes that 
with broader knowledge of zeolite diffusion processes, it has 
become clear that currently no general model for diffusivity 
prediction exists. Review of the available literature sug- 
gests that these data must, for the foreseeable future, be ob- 
tained empirically. Barrer (42) lists the variables that in- 
fluence these rates: 

1. Intracrystalline channel geometry and dimensions. 

2. Shape, size, and polarity of the diffusing molecules. 

3. Cation distributions, size, charge, and number. 

4. Lattice defects such as stacking faults. 

5. Presence of impurity molecules in the diffusion 
pathways. 

6. Structural changes brought about by penetrants. 

7. Structural changes associated with physical and 
chemical treatments. 

8. Concentration of diffusant within the crystals. 

Barrer (6) lists methods by which direct measurements 
of mass transport into or out of porous crystals can be ob- 
tained: 

1 . The change with time of the volume of gas or vapor 
around the sorbent, keeping the pressure constant. 



2. The change with time of the pressure of gas or vapor 
around the sorbent, keeping the volume constant. 

3. The change with time in the weight of sorbent bathed 
in a constant pressure of sorbate vapor. 

4. The volume change with time of a liquid sorbate in a 
sorption pipette connected to the sorbent via the vapor 
phase. 

5. The transfer with time of tracer between gas or liquid 
phase and intracrystalline sorbed phase. 

The mobilities or diffusion coefficients are inferred by ex- 
amining properties related to sorbate content or mobility 
(6). These include 

1. Birefringence changes in single crystal plates that 
parallel changes of sorbate content. 

2. Sorbate mobility inferred from jump times derived 
from nuclear magnetic resonance studies. 

3. Sorbate mobility inferred from dielectric relaxation 
processes. 

4. Molecular motion evaluated by neutron-scattering 
spectroscopy. 

5. Changes with time, as the amount sorbed changes, in 
the intensity of specific infrared absoptions characteristic of 
the sorbate interacting with the sorbent. 



Pore Volume 



The availability of large volumes of internal space is one 
of the most desirable characteristics of zeolites. The amount 
available depends on so many variables that these data must 
be derived empirically. Some of these variables are 

1. Micropore accessibility -the size and geometry of 
controlling apertures, and any blockage of these by guest 
ions or stacking faults, can admit or occlude prospective sor- 
bates. 

2. The size, shape, number, and location of cations. 

3. The size and shape of the sorbate molecules. 

4. Temperature. 

5. Pressure. 

Table 8, from Breck (11), illustrates the effects of ex- 
changeable cations on the available pore volume of zeolite A. 
Smaller void volumes are associated with the presence of 
large cations. For example, when large Tl cations replace 
Na cations the void volume is reduced by 30%. 

These volume measurements were determined from the 
water contained in a fully hydrated zeolite (IS). They give 
good comparative results, but if the zeolite structure con- 
tains cages inaccessible to molecules larger than water, then 

Table 8.— Effect of cation exchange on void volume 
in zeolite A (11) 



Unit cell 



Li 8 Na J (A)-24 H 2 . . . . 

Na„(A).27 H,0 

Ag, 2 (A)-24 H 2 . . . . . 
Tl„ 8 Na 2 ,(A).20 H 2 . 
Ca 6 (A).30 H;0 







total pore 


Density, g/cm 3 


vo 


ume, A 3 per 
unit cell 


1.91 




735 


1.99 




833 


2.76 




733 


3.36 




584 


2.05 




833 



15 



the effective volume is reduced (11). Of the calculated void 
volume of zeolite A, for example, 16% is unavailable for 
molecules larger than water. Table 9, also from Breck (11), 
indicates the effect of different-sized sorbate molecules on 
void volume determinations. For larger or branched 
molecules, less void space is accessible and fewer molecules 
can be adsorbed per unit cell. 



Table 9.— Void volume of zeolite NaX at 25° C (11) 



ZEOLITES AND CATALYSIS 



Total pore Molecules per 

Adsorbed molecule volume, A 1 per unit cell 

unit cell 



H,0 7,980 

n'pentane 6,581 

Neopentane 5,860 

2,2,4,-trimethylpentane 6,006 

Benzene 6,609 

(C.HjJjN 6,490 



265 
34 
29 
22 
45 
16.4 



Sleight (43) defined "a catalyst as a substance that ac- 
celerates a chemical reaction without itself being consumed. 
The degree of acceleration can be enormous, sometimes 
more than a factor of 10 10 ." Many natural and synthetic 
zeolites are catalytic agents; many gain additional catalytic 
properties by modification and/or by acting as supports for 
other catalytic agents; and some catalyst mixtures are only 
partially composed of zeolites so as to gain both physical and 
chemical properties from the matrix material, which can 
also act as a dilutent to reduce overly high catalytic activity. 

Sleight (-45) said that "one of the most useful zeolite 
structures is that of the naturally occurring zeolite 
faujasite." Because natural faujasite is not readily available, 
synthesis is the primary source (44). The structure of fau- 
jasite is shown in figure 6. The silica-to-alumina ratio ranges 
from 1 to 3 (11). Zeolites synthesized with this structure are 
generally referred to as either zeolite X (small silicon- 
aluminum ratio) or zeolite Y (large silicon-aluminum ratio). 
The free pore openings of the faujasite structure are the 
largest among zeolites, about 7.4 A (11). 

Plank (45) reported that the invention of zeolite cracking 
catalysts was in response to a need for a stable, selective 
catalyst of high activity. He looked for a catalyst whose ac- 
tive sites were located in controlled pores not much larger 
than the molecules to be cracked. Both molecular sieve 
zeolites and silica-alumina gels were considered, but stream 
regeneration significantly lowered the activity of the gel for- 
mulation while not affecting zeolite. 

As pure zeolites had unacceptably high activities and 
needed diluting with a matrix that would add stability, the 
first experimental catalyst contained 25% sodium faujasite 
(Na X) base-exchanged with ammonium chloride in a silica- 
gel matrix (45). It gave desirable selectivity and activity and 
continued to function well at coke levels that would have 
deactivated previously used catalysts. Using ion-exchange 
mechanisms to introduce either calcium, manganese, or 
rare-earth active sites in the zeolite matrix, Plank produced 
catalysts that had at least 100 times the activity of the con- 
ventional silica-alumina catalysts. 

Plank and coworkers developed several catalysts during 
the 1960's that virtually revolutionized the refining of oil to 
produce gasoline and other products (45). Plank reported 
that, compared with conventional methods, the calcium 
catalyst produced 20% more gasoline; the manganese, 24% 
more; and the rare earth, 16% more. Coke decreased by 
28%, 56%, and 40% respectively. Plank calculated that a 7% 
increment in gasoline could be expected in the United 
States, with an added product value of $140 million (1967 
dollars) and a potential savings of 190 million bbl of crude oil 
years at the 1963 demand rate. These figures have in- 



creased in recent years with the major changes in the oil 
market. Magee and Blazek (35) reported that, in the decade 
between the midsixties and the midseventies, between 
10,500 and 15,000 st/yr of synthetic faujasite alone was used 
for the production of catalytic cracking catalysts. Clifton 
estimated that 92% of the world market for cracking 
catalysts was held by the zeolitic catalysts (16). 

Maugh (46), in his study of the state of the art of 
catalysis, reported that the shape-selective systems were 
one of the most intensively studied of the new classes of 
catalysts. He said that they were developed to remedy one 
of the principal drawbacks of other heterogeneous systems: 
lack of selectivity. In some industrial reactions, a wide spec- 
trum of products is generated. But for applications such as 
the production of high-octane gasoline, a narrow product 
range is desirable. 

Zeolites are successful because of the size of their sur- 
face pores and interior cavities, where reactions take place. 
The pore size determines which molecules can enter the 
cavities to undergo catalysis and which molecules can leave 
the cavities as a product of the catalytic reaction. Molecules 
with physical dimensions larger than the pore opening will 
be excluded from the cavities and will not be involved in the 
catalytic reactions. Catalysis products that are larger than 
the pore opening will be trapped in the cavities and undergo 
further catalysis. The cavities within the zeolite structures 
also affect the catalytic reaction because their size deter- 
mines which transition-state compounds are formed. By 
choosing zeolites with the appropriate pore and cavity sizes, 
chemists can control the reactions that occur. Zeolites are 
particularly valuable for converting linear hydrocarbons in- 
to branched ones that have much higher octane ratings: 
linear hydrocarbons readily fit into the cavities, where they 
are rearranged into branched compounds. The cavities con- 
strain the growth of these branched hydrocarbons to the 
desired size. 

Maugh (46) reported that catalysts may be made more 
selective by reducing the size of the catalyst from the con- 
ventional crystallites (microcrystals) to a well-defined 
cluster consisting of anywhere from two or three to a dozen 
or more metal atoms. Noble metals such as platinum, 
palladium, ruthenium, or rhodium deposited in the cavities 
of zeolites have been shown to improve the catalytic prop- 
erties of zeolites (46). 

Recently, Ozin (47) reported a method by which zero- 
valent metal clusters could be formed within the in- 
tracrystalline cavities of some zeolites. The metal atoms are 
introduced as solvated metal atoms that can pass through 
the crystal structure and enter the alpha cages while main- 
taining the integrity of the zeolite structure. Metal clusters 



16 



form when single atoms diffuse through the structure and 
agglomerate. The cluster sizes are controlled by the initial 
atom loading in the structure and the size of the alpha cage. 
This technique was proven to be a nondestructive means of 



depositing zero-valent metal atoms in the zeolite structure. 
Zeolites containing site-specific metal clusters may be 
potentially useful as catalysts for oil, gas, and petroleum 
production. 



ECONOMIC CONSIDERATIONS 



Natural zeolites have had only limited commercial suc- 
cess because of the widespread use of their synthetic 
counterparts before the existence of extensive deposits of 
natural zeolites was known. Preference was also given to 
synthetic zeolites because they are monomineralic; have a 
single type of cation with predictable ion-exchange 
capacities; have a given pore, channel, and cavity size with 
few stacking faults; and have few impurities. 

The natural zeolites would seem to have great potential 
because of a single advantage, economy. There is an approx- 
imate 4-to-l differential between the synthetic sodium A 
(Na A) available at $500/st and natural clinoptilolite at ap- 
proximately $125/st. Comparison of natural versus syn- 
thetic zeolite prices is complicated by many factors. The Na 
A is produced by companies that do not have to recover 
research costs (the original patents have expired), and there 
is a very competitive market for the detergent builder 
business, considered the most promising large user. The 
price for the natural zeolites cannot be considered repre- 
sentative, either, as prices during a market development 
period can vary from high, reflecting the costs of custom 
mining, to low, reflecting a general market. Mining costs 
have not been established for zeolite mining operations in 
the United States. 

Some of the inherent costs of synthetic zeolites cannot 
be quantified. Synthetic zeolites are produced in a slow, ex- 
pensive, energy-consuming batch process. To achieve 
homogeneity of physical and chemical characteristics re- 
quires pure, raw materials, and there are considerable 
energy expenses in the synthesis and dehydration of syn- 
thetic zeolites, as well the tailoring processes for specific ap- 
plications. 



The natural zeolites, however, occur in layered sedimen- 
tary deposits mined from open pits. The overburden and the 
ore can usually by removed without blasting. Comminution 
of zeolites requires less energy than do other minerals, and 
the occurrence of zeolites in arid climates simplifies their 
dehydration. This provides an economic advantage over 
synthetic zeolites, which should permit their increased 
utilization in the mass markets where the costs of the syn- 
thetics would be prohibited. 

An indepth analysis of the reasons for which the natural 
zeolite industry achieved lower success than expected was 
given by Eyde and Eyde in 1984 (48). They indicated that 
zeolites are being improperly marketed. Natural zeolites are 
being sold as a replacement for materials that are in some 
cases both better for the use and cheaper than natural 
zeolites. To be successful, marketing approaches should 
treat the zeolites as "speciality chemicals" and address very 
specific markets that require their special properties. For 
example, zeolites have been used to remove radioactive 
cesium and strontium from low-level wastes from nuclear 
installations (49). They have also been used as dietary sup- 
plements for poultry and swine, as fertilizers to retain am- 
monium in the soil, and for stack-gas cleanup. These are 
uses that, while limited in volume, require continuous sup- 
plies of zeolites. Many uses are ideally suited for zeolites but 
represent little growth potential. One example is using 
clinoptilolite to remove ammonia from waste waters. 
Clinoptilolite was so efficient at removing ammonia from 
the effluent at waste water treatment plants that frequent 
recharging was unnecessary and the envisioned large and 
recurring market did not develop. 



APPLICATIONS 



According to Mumpton (49), zeolites were first used 
2,000 yr ago for building stones. The ion-exchange capabili- 
ty of zeolites was first investigated about 100 yr ago; the 
molecular sieving capability for separating gases, 40 yr ago; 
and the first commercial uses of synthetic zeolites, 30 yr 
ago. Breck (50) reported that, in 25 yr, zeolites had achieved 
worldwide recognition as evidenced by the appearance of 
about 1,000 publications a year, and the recognition of 40 
natural zeolite minerals and over 150 synthetic zeolites. 
Still, in 1977, only 10% of the known natural zeolites had 
commercial applications and fewer than 10% of the syn- 
thetic zeolites were successfully marketed (50). 

Flanigen (51) reported that the major use of natural 
zeolites is in bulk mineral applications: in Europe in the 
building and construction industry, where proximity to 
building location makes them cost effective, and in the Far 
East as filler in the paper industry, largely because of the 
unavailability of alternate mineral resources. A modest 
market for zeolite minerals has developed as a molecular 
sieve adsorbent in acid gas drying in the natural gas in- 
dustry, in NH 4 removal in water treatment systems by ion 



exchange, and in the production of oxygen and nitrogen via 
adsorptive air separation, especially in Japan. In general, 
however, their penetration into molecular sieve applications 
has been quite limited. Zeolite applications are summarized 
as follows (11): 

Adsorption: 
Regenerative processes: 

Separations based on sieving. 

Separations based on selectivity. 
Purification. 
Bulk separations. 
Nonregenerative process: 

Drying. 

Windows. 

Refrigerants. 
Cryosorption. 
Ion exchange: 
Regenerative processes: 

NH 4 + removal. 

Metal separations, removal from waste water. 



17 



Nonregenerative processes: 

Radioisotope removal and storage. 

Detergent builder. 

Artificial kidney dialysate regeneration. 

Aquaculture - NH 4 + removal. 

Ruminant feeding of nonprotein nitrogen (NPN). 

Ion-exchange fertilizers. 
Catalysis: 
Hydrocarbon conversion: 

Alkylation. 

Cracking. 

Hydrocracking. 

Isomerization. 
Hydrogenation and dehydrogenation. 
Hydrodealkylation. 
Methanation. 

Shape-selective reforming. 
Dehydration. 
Methanol to gasoline. 
Organic catalysis. 
Inorganic: 

Hydrogen sulfide oxidation. 

Ammonia reduction of nitric oxide. 

Carbon monoxide oxidation. 

H 2 - 2 + H 2 . 

Some experimental work was performed in the 1960's 
using zeolites to remove cesium and strontium from water 
(52). Breck (50) postulated that zeolites could be used com- 
mercially for the removal of the radioisotopes of cesium and 
strontium by ion exchange. In 1979, a mixture of synthetic 
and natural (chabazite) zeolites were used to remove those 
radioisotopes from contaminated water at the Three Mile 
Island nuclear power plant in Pennsylvania after cooling 
waters were contaminated with radioactive materials. 

The specificity of clinoptilolite for ammonium has 
already been proven in water treatment, particularly for ac- 
quaculture. This specificity also may be useful for en- 
vironmental control during uranium processing (53). Its pur- 
pose would be to adsorb ammonia that has contaminated 
ground water as the result of the leaching of uranium ores. 

The natural zeolites have a pronounced selectivity for 
molecules with permanent dipoles, such as water (11). 
Reducing the silica-alumina ratio during synthesis or 
through acid leaching decreases the water absorption 
capacity, as shown in figure 17 (50). Silicalite, the ZSM-5 
molecular sieve polymorph, has no aluminum, has no active 
sites, and is hydrophobic (50). ZSM-5 can be formulated to 
contain enough aluminum to be active but little enough to 
remain hydrophobic. Such zeolites, then, can remove trace 
organics such as n-hexane from water but have greater 
stability during regeneration than the competing carbon. 

The Mobil Corp.'s methanol- to-gas (MTG) process using 
its ZSM-5 zeolite now has an operating plant in New 
Zealand (54). This plant has a fixed-bed reactor. The 
fluidized-bed process for MTG conversion has been declared 
successful after the completion of the final phase of opera- 
tion at a pilot plant at Wesseling, Federal Republic of Ger- 
many (55). The raw gasoline yield is 90% with an octane 
rating of 90.25. The fluidized-bed process, shown in figure 
18, is reported to significantly improve the MTG process 
and possibly permit feedstocks other than methanol to be 
used. 

Meisel (56) reported that Mobil Corp.'s ZSM-5 catalyst is 
the most versatile of all the shape-selective catalysts. He 
predicts many future application arising from its unique 
characteristics. He reports that it has all the right proper- 




Si0 2 -Al 2 3 RATIO 

Figure 17.— Adsorption of cyclohexane and water on 
dealuminized mordenite. (Reprinted with permission from 
Hydrophobic Properties of Zeolites (p. 60), by N. Y. Chen. J. Phys. Chem., 
v. 80, No. 1. Copyright 1976, American Chemical Society.) 

ties: catalytic activity, which may be tailored for a given 
reaction; exceptionally long life; and a shape- selective pore 
system whose channels closely match the size of molecules 
common to numerous petrochemical processes. Applications 
for ZSM-5 catalyst range from the "building down" of 
molecules, the scrambling of molecules in various rear- 
rangement processes, and the coupling of two different 
molecules, to the building up of smller molecules to larger 
ones. 

Meisel reported that 10 of the catalyst processes using 
ZSM-5 had been identified and developed in the short span 
of 10 yr. Five of these had been scaled up and were 
operating commercially in 35 reactors throughout the 
world. Several of the remaining five exhibited excellent 
commercial potential. They collectively represent an 
unusual variety of refining and petrochemical processes and 
may reflect only a fraction of the potential of zeolite 
catalysts. 

Besides the well-documented uses of zeolites as a desic- 
cant, ammonium remover, and gas stream dryer, Garg and 
Ausikaitis (5T) reported the use of zeolites in energy- 
efficient adsorption cycle for removing large amounts of 
water (up to approximately 20%) from process streams. The 
adsorption cycle and the unique properties of zeolite 
molecular sieves can be used to dehydrate water-organic 
azeotropes. The dehydration of the ethanol-water azeotrope 
can be accomplished with less capital and lower energy costs 
[less than 2,000 Btu/gal (560 kJ/L)] using zeolites than with 
conventional azeotropic distillation methods. They claimed 
that this technology extends the use of molecular sieve ad- 
sorptive dehydration to the removal of over 20 wt % water 
from organic admixtures, and is capable, for example, of 
dehydrating the ethanol-water azeotrope from 190 proof 
(7.58 wt % water) to 199 proof ethanol. 



18 



Vent 




Condenser 



Cooler 



Vaporizer 



Superheater 



Separator 




Figure 18.— Fluidized-bed methanol-to-gasollne process. (Reprinted with permission from First Methanol-to-Gasollne Plant Nears Startup 
In New Zealand (p. 40), by J. Haggln. Chem. and Eng. News, v. 63, No. 12. Copyright 1985, American Chemical Society.) 



In describing their process, Garg and Ausikaitis said 
that it uses a fraction of the energy required by conventional 
thermal-swing regeneration schemes. This energy savings 
is accomplished by operating in a manner that causes almost 
all of the heat of adsorption to be stored as a temperature 
rise within the bed during the adsorption step. The adsorp- 
tion step uses the stored heat for regeneration energy and 
reduces the volume and temperature of the regeneration 
purge gas required. 

Natural zeolities may prove a useful energy storage 
medium for solar energy for either cooling or heating {57). 
Energy may be stored using a process that resembles a 
condensation-evaporation phenomenon. When gas is ad- 
sorbed on a solid, heat is always released, while the reverse 
process requires heat input. In a water-zeolite system, the 
zeolite is first dried by passing hot air from a solar collector 
through a container holding zeolite. The zeolite heats up, 
releasing adsorbed water, and the air becomes saturated 
with water. By using a suitable heat exchanger, this water 
vapor may be condensed and the heat of condensation ex- 
tracted. After the zeolite has been dehydrated, it has the 
potential for producing heat. To extract heat, moist ambient 
air is driven through the zeolite bed. The water vapor in the 
air is adsorbed by the zeolite, releasing the heat of adsorp- 
tion, with the result that warm dry air is produced. It is 
significant that even when the air is saturated with water 
vapor, airflow is never restricted. After the drying cycle, 
the zeolite may be allowed to cool down to room 
temperature to store energy indefinitely, provided that 
water vapor is excluded. This provides a decided advantage 
over other systems that require efficient insulation to main- 
tain temperatures as high as possible even when heat is not 
being extracted. 

Tchernev (58) explains that zeolites provide a unique op- 
portunity to exploit solid-gas adsorption cooling systems 
because of their extremely nonlinear adsorption isotherms. 



He had demonstrated the feasibility of using a zeolite 
system to provide domestic hot water and space heating 
with overall efficiencies above 75% and space cooling with 
an overall efficiency above 50%. The system uses natural 
chabazite or clinoptilolite as the solid adsorber and water 
vapor as the working fluid. Figure 19 is a diagram of his 
system. 




Figure 19.— Combined fluid-gas adsorption cooling and 
heating system utilizing zeolites. (Reprinted with permission from 
Solar Energy Application of Natural Zeolites (p. 481), by D. I. Tchernev. 
Ch. In Natural Zeolites: Occurrence, Properties, Use, ed. by L. B. Sand 
and F. A. Mumpton. Copyright 1978, Pergamon Press.) 



19 



In another zeolite-water interaction, a two-step thermo- 
chemical cycle for the decomposition of water using strong 
ionizing property of zeolites has been reported (59). Zeolites 
will induce electron transfer reactions between molecules 
and reduce multivalent cations of adsorbed molecules. A 
reaction cycle using the Cr 3+ "Cr 2+ couple in mordenite was 
devised. Upon dehydration of the Cr 3+ mordenite, reduc- 
tion to Cr 2+ occurs, with the evolution of oxygen. Upon 
rehydration, the divalent chromium oxidizes to Cr 3+ , with 
the evolution of hydrogen. Similar results were obtained 
with an In 3 + -exchanged mordenite. The zeolite used was a 
hydrogen form of synthetic mordenite. 

The initial major uses for natural zeolites may be for 
agriculture-aquaculture areas. The economic advantages of 
the natural minerals for large tonnage uses would seem to 
dictate their usage here. The present status in that area was 
best expressed by Mumpton in the abstract of his keynote 
address (53) at the international conference, "Zeo- 
Agriculture '82": 

Following earlier work in Japan and other countries 
numerous studies have been carried out in recent years in 
the United States and abroad on the agriculture potential of 
zeolite minerals with considerable success, and with some 
failures. Based on their attractive ion-exchange, adsorption, 
and hydration properties, natural zeolites have been found 
to act as slow-release fertilizers to provide potassium and 
nitrogen to agricultural soils, as carriers of herbicides, 
fungicides, and insecticides, as possible traps for heavy 
metal contaminants in soils amended with municipal 
sewage, and as decaking agents for fertilizer and feed 
storage. Their addition to the normal diets of swine, poultry, 
and ruminants has resulted in increased body weights and 
feed efficiencies, with simultaneous decrease in the incidence 
of scours and other intestinal diseases, especially among 
young animals. 

Zeolites have also been shown to deodorize and increase 
the nutrient content of animal excrement, thereby improving 
the fertilizer value of the manure and lessening the energy re- 
quired for ventilation of poultry houses and other buildings 
for confined livestock. Zeolite filtration has been employed to 
remove toxic concentrations of ammonia from aquacultural 
systems, and zeolite adsorption processes have been 
developed to provide oxygen-enriched air for such systems, 
both of which allow more fish to be raised or transported in 
the same volume of water. Preliminary ecperiments even 
suggest that adding zeolites to fish rations can increase 
biomass production. The hydration-dehydration properties 
of certain zeolites have been harnessed in the development of 



solar-refrigeration units to preserve dairy and other animal 
products and veterinary supplies in areas where electricity 
is not readily available. 

Although many of these investigations are preliminary, 
and many have failed to reproduce the results obtained in 
other laboratories, the experimental evidence to date strongly 
suggests that continued investigation under controlled condi- 
tions will lead to numerous areas where natural zeolites can 
contribute to increased animal and crop production. The 
adage that "where there's smoke, there's fire" will undoubt- 
edly hold true for the emerging field of zeo- agriculture. 

Flanigen (51) reported that "future trends in materials 
will no doubt see the development of new commercial 
zeolites selected from newly discovered compositions and 
structures, chemical modifications of present commercial 
products to generate new and useful properties, and a 
reevaluation of the host of known zeolites which never 
achieved commercial success. It seems likely that with the 
increasing number of laboratories devoting resources to the 
search for new structures and compositions, new classes of 
molecular sieve materials will be discovered. The modifica- 
tion chemistry of zeolites, practiced to date, such as steam- 
ing and chemical extraction, leaves a vast area of chemical 
and structural modification of solids as yet unexplored with 
zeolites." 

She also predicts that "additional types of natural 
zeolites will probably not achieve commercial prominence 
since the large geological exploration efforts for zeolite 
deposits throughout the world during the last ten to fifteen 
years have probably identified all of the zeolite mineral 
species of commercial significance" and that "natural 
zeolites should continue to grow as an important industrial 
mineral resource used principally in bulk application areas. 
The 'engineering 1 of the mined zeolites, by beneficiation to 
upgrade purity and chemical modification to tailor proper- 
ties, will no doubt emerge as the level of technically inten- 
sive effort on natural zeolites expands." 

As a last note, it is interesting that the creators of the 
synthetic zeolite industry have themselves created a com- 
petitive product for at least the molecular sieve portion of 
their market (60). The Union Carbide Corp. scientists 
learned enough about porous crystals from their pioneering 
zeolite research to enable them to discover and develop a 
family of aluminophosphate and silica-aluminum phosphate 
crystalline solids. These microporous solids have slightly dif- 
ferent physical and chemical properties from zeolites and 
have potential uses as molecular sieves. 



SUMMARY 



Zeolites are crystalline aluminosilicates of the alkaline 
and alkaline-earth metals. They possess many desirable ion- 
exchange, molecular sieving, and catalytic properties, which 
make them valuable mineral commodities. It is only within 
the past 40 yr that zeolites have been used extensively in 
commercial processes. The petroleum industry, for exam- 
ple, has profited immensely from the use of zeolites in refin- 
ing operations. Ironically, research on the properties of 
natural zeolites created the synthetic zeolite industry. Com- 
mercial applications for natural zeolites, which were con- 



sidered to be mineralogical curiosities, were discovered, so 
the emphasis during the 1940's and 1950's was on the com- 
mercial production of their synthetic counterparts. Syn- 
thetic zeolites were firmly established in the market when 
large deposits of natural zeolites were discovered in the 
1950's and 1960's. Only recently have natural zeolites begun 
to be used commercially, not as replacements for synthetic 
zeolites but in areas where the use of synthetic zeolites 
would prove uneconomical. 



20 



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21 



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